The present invention relates to a transmission control system for an infinitely variable belt-drive automatic transmission for a vehicle, and particularly to a system which improves the transmission characteristic during acceleration in a drive range (D-range).
The infinitely variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley depending on driving conditions. The system is provided with an oil pressure regulator valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The primary pulley has a pitot pressure generating device as an engine speed sensor for producing pitot pressure dependent on engine speed. The pitot pressure increases with an increase of engine speed and is applied to one axial end of the spool of each valve to bias the spool. The pressure regulator valve is so arranged as to decrease the line pressure of a hydraulic circuit with an increase of the pitot pressure and a decrease of the transmission ratio. At the other end of the spool of the transmission ratio control valve, a spring load is applied which is dependent on the depression position of an accelerator pedal of a motor vehicle. Thus, the spool of the transmission ratio control valve is positioned at a location where the pitot pressure and the spring load balance so as to decide the transmission ratio depending the engine operating conditions. At the idling operation of the engine, line pressure is regulated at the highest pressure by the regulator valve and the line pressure is not applied to the servo device of the drive pulley. Accordingly, the transmission ratio is set at the highest value. Under such a condition, when engine speed increases with the depression of the accelerator pedal, the motor vehicle is started by the engagement of an electromagnetic clutch. When the engine speed reaches a predetermined speed (for example a point B in FIG. 4), the spool of the transmission ratio control valve is shifted in the upshift direction by increased pitot pressure, so that the transmission ratio starts to change to decrease the ratio (upshifting). If engine speed is kept constant, the transmission ratio continuously decreases. Accordingly, vehicle speed increases, even if the accelerator pedal is not depressed. At the lowest transmission ratio, when the depression of the accelerator pedal is decreased, the vehicle speed decreases at the lowest transmission ratio with decrease of the engine speed. When the engine speed reaches a predetermined low speed, the transmission ratio starts to a change to higher ratio (downshift). Accordingly, if the engine speed is kept constant, the vehicle speed decreases.
As described above, the vehicle speed changes at a constant engine speed by automatic upshifting and downshifting of the transmission.
However, such a constant engine speed during the transmission changing operation is unsuitable for driving the motor vehicle. More particularly, the engine speed is comparatively high at a high transmission ratio, and is too low to actively drive the motor vehicle at a low transmission ratio. Japanese Patent Laid Open 56-66553 (U.S. Pat. No. 4,400,164) discloses an infinitely variable transmission which may resolve the above problems. The transmission is characterized by a regulator plunger (67 FIG. 2 of this U.S. patent) slidably mounted in the spool of the transmission ratio control valve and a regulator spring provided between one end of the regulator plunger and one end of an actuating rod which is operatively connected to an accelerator pedal of a motor vehicle so as to urge the regulator plunger in accordance with the depression of the pedal. The regulator plunger has a flange subjected to the line pressure controlled by the regulator valve.
When the transmission is upshifted, the regulator plunger is projected by the decrease of the line pressure to increase the load of the regulator spring. Accordingly, the pitot pressure for shifting the spool of the control valve in the upshifting direction is elevated. Consequently, unless the engine speed is increased, the transmission is not upshifted. Thus, the transmission characteristic can be shown by lines A-B-C-D-E-A in FIG. 4. In the this figure, line l is the largest transmission ratio line and line h is the lowest ratio line. The transmission is upshifted with an increase of the engine speed. However, since the increase of load of the regulator spring is dependent on the line pressure, the engine speed does not increase in proportion to the increase of the vehicle speed as shown by the line m.sub.1. Accordingly, if the upshift starting point B is set at a position where the largest torque of the engine is obtained, the highest engine speed at point C does not reach the largest horsepower speed.
On the contrary, since the regulator spring is adjusted so that the largest horsepower can be obtained at point C, the engine speed at point B becomes extremely high, which results in a decrease of acceleration characteristic of the vehicle.